Investiagtion On The Process,Microstructure And Properties Of Selective Laser Melting QCr0.8 And Cu10Zn Alloy

Copper alloys are widely used in power electronics,aerospace,and petrochemical industries due to their excellent electrical and thermal conductivities,outstanding corrosion resistance,and ease for fabrication.With the strong demand of lightweight and the integration of structure and function in the industry,the shape of components is becoming more and more complex.However,conventional processing methods are difficult to produce components with complex shapes.Selective laser melting(SLM)is a rapidly developed 3D printing technology in recent years.If SLM can be used to produce copper alloy components,copper alloy components with very complicated shapes and high performance will be produced.However,due to the extremely high laser reflectivity and thermal conductivity,there are a few reports on the SLMed copper alloys,especially for Cu Cr alloy and brass.This thesis focuses on the melting behavior,densification evolution,microstructure and mechanical properties of SLM Cu10Zn and QCr0.8 alloy.The SLMed copper alloy parts with high density and mechanical properties are achieved.The main conclusions are as follows:(1)Through theoretical calculation of the center temperature of the SLMed Cu10Zn and QCr0.8 alloy single track,the lifetime of molten pool and melt spread time,the relationship between the melting behavior and the process parameters are revealed.The optimal process parameters are obtained for fabricating the stable and regular single track.Compared with SLMed Fe-base alloy and aluminum alloy,SLMed Cu10Zn alloy and QCr0.8 alloy require high laser power to melt the metal powder when same material substrate is utilized.Under the same process parameters,the lifetime and melt spread time of SLMed Cu10Zn alloy single track are larger than those of SLMed QCr0.8 alloy single track.For same laser power,the critical scanning speed of Cu10Zn alloy to obtain a stable single track is slightly smaller than that of QCr0.8 alloy.Due to the recoil pressure of the molten pool caused by the vaporization of Zn element,the surface roughness of Cu10Zn single track is larger than that of QCr0.8 alloy.Reducing the laser power can effectively reduce the recoil pressure and achieve smooth single track.(2)The relationship between the process parameters and metallurgical defects of SLMed Cu10Zn and QCr0.8 alloys is studied.It is found that the metallurgical defects are pores without cracks.When the laser energy is large,the pores are nearly circular;when the laser energy is small,the pores are irregular.Finally,99.9%relative density is achieved for SLMed Cu10Zn and QCr0.8alloy.(3)The microstructure of SLMed QCr0.8 alloy consists of columnar grains growing along the forming direction,very fine cellular sub-structures and precipitates.The phase composition is?-Cu,Cr and Cr₂O₃.With the increase of the scanning speed,the solid solubility of Cr increases;the grain size decreases;the preferred orientation of grain parallel to z direction gradually changes from<1 0 0>direction to<1 1 0>direction.Compared with the annealed sample after forging,the tensile strength and yield strength of SLMed QCr0.8 alloy are higher,but the elongation and conductivity are lower.The microstructure of SLMed Cu10Zn alloy is columnar grain growing along the forming direction,the phase composition is only?-Cu.The grain orientation is<10 0>direction.Compared with the annealed sample after forging,the tensile strength of the SLMed sample is higher than 10%,the elongation is lower than 25%,and the electrical conductivity is almost equivalent.(4)The effect of heat treatment on the microstructure and properties of SLMed QCr0.8 alloy is systematically investigated.Two different effects of the solution heat treatment(SHT)temperature on the microstructure of SLMed QCr0.8 alloy happen.A SHT temperature threshold exists.When the SHT temperature is higher than the threshold,the Cr particles can dissolve into Cu matrix.In this experiment,the threshold SHT temperature is 1000^oC.When the SHT temperature is 1020^oC,complete solid solution happens.After solution and aging treatment(SHA),the dislocation is reduced and Cr phases precipitate from Cu matrix.The mechanical properties and conductivity are greatly improved.After direct aging treatment,Cr particles also precipitate,but the dislocation can be still observed.The volume fraction of the precipitates in directly aging(DA)sample is larger than that in SHA sample.Therefore,the mechanical properties and electrical conductivity are the best after direct aging,and the tensile strength,yield strength,elongation and electrical conductivity are 468.0MPa,377.3MPa,19.2%and 98.3%IACS,respectively.After solution treatment,the dislocation density in the microstructure decreased,the precipitation phase decreased,and the corrosion performance is the best.(5)The microstructure and properties of SLMed QCr0.8 alloy are controlled by using scanning strategy and adding micro-amount alloying element.With the increase of the SLM phase angle,the microstructure and grain orientation of SLMed sample change.After aging treatment,the conductivity of the samples formed by different phase angles is almost the same within the fluctuation range;since the grain size is the smallest when the phase angle is 90^o,the sample has the highest strength after aging treatment.Zr element can be added to refine the grain of SLMed QCr0.8 sample.Due to the pinning action of the Zr element,the grain size after aging treatment hardly changes,unlike SLM samples without Zr,which will not have grain boundary migration after aging treatment.When the content of Zr is in the range of 0.1wt%-0.3wt%,the average size of grain is the smallest,the microhardness and conductivity are the largest with the Zr content being 0.1wt%.After aging treatment,the microhardness and conductivity increase significantly.Compared with the samples without Zr elements,the microhardness is greatly improved,while the conductivity is decreased.